1. Field of the Invention
The present invention relates to a corona charging method and a corona charger for use in electrophotographic copying apparatus, facsimile apparatus and printers, and to an image formation apparatus equipped with a corona charger.
2. Discussion of Background
In image formation systems such as copying machines, printers, and facsimile apparatus, various methods such as electrophotographic image transfer method, thermal image transfer method, bubble jet method, and ink jet method are employed to develop latent invisible images to visible images.
Of these methods, the electrophotographic image transfer method is most widely used, in which electrically charged toner particles are transferred imagewise to an image transfer sheet. More specifically, in a representative electrophotographic image transfer method called "Carlson Process", a recording medium such as a photoconductor is electrically uniformly charged in the dark, and then exposed to a light image so as to form a latent electrostatic image on the recording medium. The above-mentioned charged toner is then deposited on the latent electrostatic image to form a visible toner image corresponding to the latent electrostatic image on the recording medium. The thus formed visible toner image is then transferred to an image transfer sheet such as a sheet of paper.
In such electrophotography, there is a charge transfer phenomenon in each of a charging process, development process, image transfer process, and charge quenching process. In order to have charges transferred subsequently in each of the above-mentioned processes, it is necessary to cause the generation of charges and the transfer of charges to a photoconductor, a charge bearing material, or an image transfer or receiving material. In other words, in order to perform the transfer of charges, an electric treatment called "charging" is carried out.
In order to perform such "charging", for instance, contact triboelectric charging, contact charge injection, and radiation ionization can be employed. Contact triboelectric charging and contact charge injection are carried out using a development roller and a charging roller.
However, when a charging member, such as a development roller or a charging roller, comes into contact with a chargeable member which is to be charged, low-molecular weight components from such a roller are deposited on the surface of the chargeable member, so that the chargeable member is contaminated with such low-molecular weight components, resulting in causing abnormality in image formation. Radiation ionization is not currently used in practice since exposure-proof is required although it depends upon the kind of radiation to be used.
A corona charging method, which is conventionally and currently most widely used, is carried out by applying a high voltage to an electrode made of a thin wire or a stylus, and performing corona charging between the electrode and a counter electrode to generate corona ions, thereby transferring the thus generated corona ions to a chargeable member or charging the chargeable member.
The principle of this corona charging method is simple and an apparatus for this method is also simple in mechanism. However, oxygen is contained in an amount of about 40% in volume in air, so that when corona charging is carried out in air, the oxygen in the air is ionized to generate ozone (O.sub.3) Ozone is a poisonous substance, so that the generation thereof in offices is not preferable.
Furthermore, when corona charging is carried out in an atmosphere of nitrogen, corona ions change nitrogen into nitrogen oxides such as NO.sub.x. When the thus formed NO.sub.x is deposited on the surface of a photoconductor, the hygroscopicity of the photoconductor is so increased that the charging performance of the photoconductor is caused to deteriorate significantly. Under such circumstances, some other improved method is desired.
One method is proposed to reduce the generation of ozone, in which charge injection is carried out, with a charging member being disposed in contact with a chargeable member, or corona charging is carried out between the charging member and the chargeable member which are closely disposed, utilizing Paschen's law, and begins to be used.
However, the above-mentioned contact charging method basically uses corona charging, so that as long as charging is conducted in an air-containing atmosphere, the generation of ozone is inevitable. Under such circumstances, a method of injecting an inert gas such as argon in the contact charging method has been proposed as disclosed in Japanese Laid-Open Patent Application 59-204057, and also a method of using a gas with an oxygen content thereof being less than that of air has been proposed to minimize the generation of ozone as disclosed in Japanese Laid-Open Patent Application 60-95459.
As a matter of fact, the generation of ozone can be reduced as the content of oxygen in the gas is reduced. However, the generation of ozone cannot completely be prevented. Therefore some complicated mechanism, such as a mechanism in which a gas separation filter is used, is inevitably required.
Furthermore, as long as the corona charging phenomenon is used, a luminescence phenomenon inevitably takes place simultaneously with the corona charging. This luminescence phenomenon can be easily observed in a dark room.
A latent electrostatic image is formed on a photoconductor with such a mechanism that the photoconductor, which is a dielectric material, is uniformly charged in the dark, and the uniformly charged photoconductor is then exposed to the light of a light image, so that the portions exposed to the light image of the photoconductor become electroconductive and accordingly the charges in the portions exposed to light dissipate away through a ground, or the charges in the portion exposed to light are neutralized by the electric charges with a polarity opposite to the polarity of the charges therein, injected thereto from the side of the ground, whereby a latent electrostatic image corresponding to the light image is formed on the surface of the photoconductor. In the course of the charging process with the above-mentioned mechanism, the occurrence of the luminescence phenomenon adversely reduces the charging effect. In conventional image formation apparatus such as copying machines, the occurrence of the luminescence phenomenon is not a problem because the luminescence is extremely slight in view of the quality of images to be produced. However, recently the formation of extremely high quality images with high speed is desired and accordingly highly photosensitive photoconductors are being used. Under such circumstances, the adverse effects of the above-mentioned luminescence phenomenon cannot be ignored.